The present invention relates to an optical display for head mounted displays, and more particularly, to a two-stage optical system for this display that comprises a first stage for magnification and image sizing and a second stage that includes a total internal reflection process.
While the software and computer hardware for creating virtual reality has continued to improve, there still exists a need for physically presenting a visual display and audio signals to the user. Such a device is shown in U.S. Pat. No. 4,952,024 which disclosures center rib head mounted display. Alternative, devices have employed a helmet that includes a mounted visor that reflects projected light to present a virtual display to the wearer.
Generating sizable displays (displays that measure upwards of 2 inches diagonally) is expensive and greatly increases the cost of a head mounted display. Head mounted displays necessarily present the viewer with virtual images. In order to provide a viewer with as complete a virtual reality experience as possible, the image he sees should fill his field of vision. The viewer must also be able to look around at his environment. In order to accomplish these goals, a head-mounted display needs to provide a virtual image to the viewer as opposed to a real image.
A real image refers to an image that is observed directly by the unaided human eye. A photograph is an example of a real image. Electronic displays that provide a real image generally provide some form of display surface on which the real image is formed and viewed. A real image can be observed by the unaided eye when a viewing surface is positioned at its location. Examples of electronic displays that provide real images include liquid crystal displays, CRT monitors, and projection screens.
By contrast to a real image, a virtual image is an image which, if a viewing surface were positioned at the location of the virtual image, no image would be observed by the eye. An example of a virtual image is the image of fine print viewed through a magnifying glass. The print not only appears larger, it also appears to be located substantially behind the surface where the print actually exists. By definition, a virtual image can exist at a location where no display surface exists. The size of the virtual image therefore is not limited by the size of a display surface. Virtual image displays thus have the advantage of eliminating the need for a large display surface in order to provide a large image to the viewer.
A virtual image display must initially form a source object that is then imaged by an optical system to create the virtual image. A substantial advantage of a virtual image electronic display is that the source object initially created may be as small as can be usefully reimaged by the optical system. As a result, virtual image electronic displays may effectively utilize very small displays to form the source object. Pixel sizes may be as small as a few microns in diameter. At this size, the unaided eye cannot resolve images. Rather, in order to view the source object formed by the display, substantial magnification of the optical system is required.
A virtual image must be created by an optical system of some kind. In a real image electronic display, it is the eye and the viewing surface properties that determine the viewing parameters. By contrast, in a virtual image display, the optical system determines most of the viewing parameters.
There are three important parameters relating to the ease of viewing the image associated with virtual image displays. The first parameter is the far point. This refers to the maximum distance from the eye which the optical system can be held and have the eye still see the entire virtual image. Optical devices which provide a far point which is a short distance from the optic are undesirable due to the inconvenience and discomfort associated with placing the eye in close proximity with the optic. It is therefore preferred that an optic provide a long far point in order to enable the magnified image to be viewed through the optic at a comfortable and convenient range of distances from the optic.
The second parameter relating to the ease of viewing a virtual image is the apparent angular width of the virtual image, commonly referred to as the field of view of the virtual image. The full field of view is defined as the ratio of the largest apparent dimension of the virtual image to the apparent distance to the virtual image. It is generally equivalent to the field of view for a real image display surface.
The third parameter relating to the ease of viewing a virtual image is the transverse distance that the eye may move with respect to the optical system and still have the eye see the entire virtual image through the optical system. This is commonly referred to as the xe2x80x9ceyebox.xe2x80x9d
A need currently exists for an inexpensive, compact virtual image display for a head mounted apparatus that is positionable within a small volume, that provides the observer with a large field of view, a virtual image with a significant degree of eye relief and a large translational distance.
It is recognized that one of the primary factors driving up the cost of head mounted displays is the cost of the initial display. Prior art head mounted displays have been created that use smaller displays coupled to magnification systems. These generate the larger virtual images the viewer sees. However, prior art magnification processes are bulky and can make head mounted displays unwieldy and cumbersome.
Therefore, the need exists for a lightweight head mounted display that operably presents a visual display occupying all or almost all of the viewer""s field of vision to a wearer that is both comfortable and relatively inexpensive.
Further, video images are recorded with various aspect ratios, e.g., 4:3 and 16:9. The display screen showing the unmagnified video image will necessarily have fixed dimensions. Therefore, for all images with aspect ratios that do not match the fixed dimensions of the display screen, there will be distortion in either the height or the width of the image dependent upon the relation between the aspect ratio of the image and that of the display. It would be an improvement if the image could be adjusted so that it appears undistorted to a viewer.
An object of the invention is to create a head-mounted display that is comfortable, lightweight, and relatively inexpensive.
Another object of the invention is to create a head-mounted display with a lens that is adjustable so that regardless of the aspect ratio of the initial image, the image that is ultimately presented to the viewer fills the viewer""s field of vision.
This and other objects of the invention are accomplished by a head mounted display having a two-stage optical system where the second stage magnification is accomplished using total internal reflection techniques. This two-stage system is usable in a relatively compact and inexpensive head mounted display. In a preferred embodiment, the head-mounted display has two sections extending rearward around the sides of the head. Within each section, a display screen projects an image that passes through a first lens that adjusts the size of the image. It can magnify the image, alter the aspect ratio of the image, or both. The image then undergoes total internal reflection within a subsequent lens, resulting in magnification of the image. The viewer is presented with a virtual image many times larger than the original display.